Polyester, especially having an ethylene-terephthalate recurring-unit as a main unit, is excellent in chemical resistance, heat resistance and light resistance, and shows excellent characteristics in electric, mechanical and physical properties. Therefore, the polyester is widely employed for the preparation of a film, a fiber or a mold. Such polyester is prepared by the steps of esterifying terephthalic acid with ethylene glycol to obtain glycol ester of terephthalic acid and/or a low molecular weight polymer thereof, and polycondensating the ester and the low molecular weight polymer in the presence of a polymerization catalyst.
As examples of the polymerization catalyst, there have been known metal-containing compounds such as an antimony compound, a germanium compound, a titanium compound and an organic tin compound, and nonmetal compounds such as an organic sulfinic acid compound. The antimony compound, especially antimony trioxide, is generally employed from the viewpoint of polymerization activity, low coloration of the resulting polyester and cost performance. Although the antimony compound exhibits an excellent polymerization activity, the compound is deposited in the course of synthesis of polyester to form its particle which is insoluble in the resultant polyester.
A polyester film is prepared by the steps of extruding melted polyester in the form of sheet from an extruder, feeding the sheet-shaped polyester onto the casting drum to rapidly cool and biaxially (lengthwise and crosswise) orienting the sheet-shaped polyester to form the polyester film. In order to enhance the productivity of the polyester film, it is effective to increase a film-formation rate by increase of a periphery speed of the drum. As a process of enabling the speed of film formation to increase, there has been known an electrostatic application process which comprises preparing a uniform film by closely bonding melted polyester onto the surface of the casting drum of an extruder, the extruder being provided with an electrode in the form of wire between an extruder head and the surface of the casting drum (Japanese Patent Publication No. 37-6142).
In order to enhance the film-formation speed utilizing the electrostatic application process, there have been proposed processes for preparation of polyester film such as a process employing an alkali or alkali earth metal or its compound in the polymerization step (Japanese Patent Provisional Publication No. 51-70269) and a process employing an ethylene glycol-soluble magnesium compound or manganese compound and phosphorous acid (or its salt) or phosphoric acid (or its salt) in the polymerization step (Japanese Patent Provisional Publication No. 55-84322). These processes bring about enhancement of adaptability for the electrostatic application process to increase the film-formation speed because of increase of the amount of electric charge to be deposited on the surface of the polyester.
However, addition of the metal compound to the reaction materials for preparing polyester (polyester-synthesizing system) in the presence of an antimony compound as the polymerization catalyst results in deposition of black particles insoluble in the resultant polyester, the particles being metal particles derived from the antimony compound. Further, the resultant polymer is colored due to use of both the antimony compound and the metal compound. In the case that the polyester containing the above black particles is used in the film formation by means of the extruder, a filter used in extrusion of the melted polyester is clogged with the particle to increase pressure on the filter. Further, a polyester film prepared from the polyester containing the particle can be hardly utilized for the preparation of photographic films.
To solve the above problems, Japanese Patent Provisional Publication No. 1-287133 proposes a process employing a glycol-soluble magnesium compound, manganese compound or zinc compound, imidazole (or its salt) or its derivative and phosphorous compound in the polyester-polymerization step. The process brings about decrease of both the number of the particles in the resultant polyester and the coloration of the polyester. However, the film has not sufficient characteristics to be adopted for the preparation of photographic films.
As processes for decreasing the sizes or the amount of the deposited particles derived from the antimony compound in which any of magnesium compound, manganese compound and zinc compound are not used, there have been known a process employing calcium carbonate or silicon oxide in the polyester-polymerization step (Japanese Patent Provisional Publication No. 64-69623), a process employing antimony trioxide together with a specific amount of bismuth in the polymerization step (Japanese Patent Provisional Publication No. 1-185355) and a process employing antimony trioxide which has a reducing property for ethylene glycol and contains a specific amount of bismuth or selenium in the polymerization step (Japanese Patent Provisional Publication No. 3-215522).
Even when these processes are employed to the preparation of polyester in the presence of magnesium compound, manganese compound or zinc compound, the amount (or the size) of the deposited particles cannot be satisfactorily reduced.
Accordingly, any conventional processes for preparation of polyester employing an antimony compound does not give a polyester film suitable for the preparation of a photographic supports, photographic films for original printing plates or micro films in which it is needed to extremely reduce the amount of foreign matters (impurity particles) such as the above particles derived from the antimony compound in the polyester.